1. Field of the Invention
The invention relates to a semiconductor device and the method of manufacturing the same. In particular, the invention relates to a semiconductor device with a nitrified gradient layer structure and the method of manufacturing the same.
2. Description of the Related Art
During the manufacturing process of semiconductor devices, one often has to define many fine patterns. A primary method of forming these patterns employs the etching technology to copy the photo resist patterns generated by microlithography onto the material under it. Therefore, the etching technology plays a very important role in semiconductor processes.
The etching technology has two types: the wet etching and the dry etching. The biggest advantage of drying etching is its anisotropic etching, which can render a more precise etching profile. However, the dry etching equipment is more expensive and involves a vacuum system. Thus, the maintenance fee is higher. A common substitute method is wet etching. Nonetheless, the wet etching usually has a fairly high selectivity for different materials. Aside from the crystalline direction that may affect the etching rate, the wet etching is basically an isotropic etching because chemical reactions do not have any preference in orientations. The isotropic etching means that the wet etching does not only etch in the vertical direction, it also etches in the horizontal direction. The horizontal etching will result in the so-called “undercut” phenomenon that cannot accurately transfer a pattern.
For multilayer thin film transistors (TFT's), the control of etching profile is often very difficult. If the etching rates of materials in different layers differ too much, it is very easy to result in serious undercut. For example, an ordinary gate uses the AlNd/AlNdN bi-layer structure. However, the etching rates of AlNd and AlNdN differ by a factor of 6 to 7, the undercut is thus unavoidable. Therefore, people insert barriers among layers that have very different etching rates as buffering. For example, one can use AlNd/MoN/Mo as the gate or Ti/Al/MoN/Mo as the source and drain. Nonetheless, this method requires additional target materials or devices and does not allow one-time film formation. Moreover, it is likely to have the material interface problem among the layers, rendering defects.